Cinchona phase transfer catalyst

Alkylation of protected glycine derivatives is one method of a-amino acid synthesis (75). Asymmetric synthesis of a D-cx-amino acid from a protected glycine derivative by using a phase-transfer catalyst derived from the cinchona alkaloids (8) has been reported (76). 

Arai and co-workers have used chiral ammonium salts 89 and 90 (Scheme 1.25) derived from cinchona alkaloids as phase-transfer catalysts for asymmetric Dar-zens reactions (Table 1.12). They obtained moderate enantioselectivities for the addition of cyclic 92 (Entries 4—6) [43] and acyclic 91 (Entries 1-3) chloroketones [44] to a range of alkyl and aromatic aldehydes [45] and also obtained moderate selectivities on treatment of chlorosulfone 93 with aromatic aldehydes (Entries 7-9) [46, 47]. Treatment of chlorosulfone 93 with ketones resulted in low enantioselectivities. 

Table 1.12 Cinchona alkaloid-derived phase-transfer catalysts for asymmetric Darzens reactions.

Cinchona alkaloids now occupy the central position in designing the chiral non-racemic phase transfer catalysts because they have various functional groups easily derivatized and are commercially available with cheap price. The quaternary ammonium salts derived from cinchona alkaloids as well as some other phase transfer catalysts are... 

B. Lygo, P. G. Wainwright, A New Class of Asymmetric Phase-Transfer Catalysts Derived from Cinchona Alkaloids - Application in the Enantioselective Synthesis of a-Amino Acids , Tetrahedron Lett., 1997, 38, 8595-8598. 

Based on prior results where Ricci used Cinchona alkaloids as phase-transfer-catalysts, the group proceeded to look at hydrophosphonylation of imines [48], Employing the chiral tertiary amine as a Brpnsted base, a-amino phosphonates products were synthesized in high yields and good selectivities. 

Corey employed a cinchona alkaloid-derived ammonium salt 5 for the solid-liquid phase transfer catalyst, and attained 99% ee in the addition of a glycine-derived imine to 2-cyclohexenone (Scheme 6) [13,14]. 

Table 2. Chiral phase transfer catalysts derived from Cinchona alkaloids developed between 1998 and 2001...

P-Hydroxyammonium salts can react under the strongly basic reaction conditions present in many phase-transfer reactions and the newly formed products could, in principle, serve either as effective or ineffective catalysts (Scheme 10.1) [9c]. The development of a new class of chiral phase-transfer catalysts, the W-alkyl-O-alkyl cinchona quats (Bactjve in Scheme 10.1 and 30 in Scheme 10.2), resulted from detailed mechanistic studies of these systems [5p,12], These catalysts are formed by in situ deprotonation of 28 to the alkoxide 29 followed by alkylation to form the active catalyst 30. Such catalysts offer an important second site of variation (R-, in 30) for catalyst development, which has been rapidly utilized for the preparation of more effective catalysts. 

Cinchona-Derived Chiral Phase-Transfer Catalysts for Amino Acid Synthesis... 

Cinchona-Derived Chiral Poly(Phase-Transfer Catalysts) for Asymmetric Synthesis... 

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